At 36 weeks, your provider will do a vaginal swab for Group B Streptococcus (GBS) bacteria, or in my practice I trust that you can swab yourself ;) We do this because when babies develop infections during labor, they tend to be caused by this particular bacteria.
For more on GBS, I recommend reading the Evidence Based Birth article. For more reading on the role that healthy bacteria plays in your body and how it relates to childbirth, read this blog post. Please let me know if there is research that I haven't included here!
In my practice, I recommend taking a good quality probiotic and eating fermented foods during pregnancy. I haven’t found any articles demonstrating this, but I suspect that nutrition, diet, and factors like toileting-habits impact GBS rates—after all, the United States has a GBS positive rate of about 20% while in Crete it’s about 11% and another study showed a 10% rate in China.
A number of studies indicate that the use of probiotics may be helpful in reducing GBS colonization in late pregnancy, which reduces the risk of GBS infection in newborns. A 2022 systematic review in the journal Nutrients found “that the administration of probiotics during pregnancy, namely in the third trimester, was associated with a reduced GBS recto-vaginal colonization at 35–37 weeks and a safe perinatal profile. Probiotics also may be useful to counteract GBS colonization when it is already established, showing a considerable negativization rate in GBS-positive women.”
A 2005 Turkish study in Mikrobiyol Bul and a 2012 study in the International Journal of Probiotics and Prebiotics showed that strains of lactobacilli decreased GBS when they came into contact in vitro.
A 2006 study in a Scandinavian journal showed that it was possible to decrease GBS colonization by having women wear panty-liners coated with probiotics.
In another small 2014 study in the Journal of Obstetric, Gynecologic and Neonatal Nursing, the probiotic group reported lower colony counts of GBS.
In a larger 2016 study in the Taiwanese Journal of Obstetrics and Gynecology, women who were GBS positive at 36-37 weeks and who took two probiotic capsules at bedtime showed a significant reduction in GBS colonization at birth.
A 2010 study from Iran in the Journal of Biological Sciences recommended supplementing with lactobacilli-containing probiotics, though did not specify oral or vaginal use.
Studies also indicate that healthy vaginal flora reduces the risk of pre-labor rupture of membranes (PROM), which is one common cause of medical induction, and which can increase the risk of infection.
A 2015 study in PLOS One showed that cervical inflammation is more likely in people who had lower amounts of lactobacilli in their vaginas. Among people whose membranes released preterm -- a serious complication of pregnancy -- almost a third occurred in the presence of elevated levels of harmful bacteria.
Some questions that continue to arise for me:
Should we take probiotics throughout pregnancy, beginning in the first trimester, or target it to specific weeks of pregnancy in the second or third trimester?
If we should take them prior to the 36-week mark (when GBS testing is performed), how many weeks prior is early enough to be effective?
What specific strains, in what specific amounts, and in what specific dosages?
Oral or vaginal? Are they equal? How does an oral probiotic change vaginal flora?
Are there dietary changes (more fermented foods, less refined sugar, etc) that can have supportive effects on vaginal lactobacilli populations?
Do probiotics make up for a poor diet? Can they?
Should everyone take probiotics in pregnancy, or just people who are GBS+?
In short, yes, evidence supports the use of probiotics to prevent GBS colonization.
But what if you test negative at 36 weeks?
While most pregnant people who test negative for GBS at 36 weeks will remain so at 40 weeks, one study showed that 61% of GBS infections occurred in babies whose mothers tested negative but then were positive by 40 weeks. This doesn't mean that we need to institute universal antibiotics for people in labor--remember, GBS overall is rare, and the universal approach is questionable--rather, it should call for the judicious use of antibiotics when risk factors are at play. It also suggests that lactobacilli probiotics would be useful among even those who test negative for GBS in anticipation of their labors.
Because there are no risks associated with taking oral probiotics in pregnancy, and because there are potential benefits to it, my belief (and that of the midwives in my community) is that everyone can benefit from supporting their gut and vaginal microbiome in pregnancy, and at any point in pregnancy. Because the cost of quality probiotics can be high, I would recommend adding fermented foods such as daily kimchi, sauerkraut, and plain whole milk yoghurt to the diet, and/or taking a good probiotic beginning around 31-32 weeks of pregnancy.
Other interesting studies I came across...
According to this 2013 study in Acta Pediatrica, the use of antibiotics in labor decreased the transmission of healthy vaginal lactobacilli to the baby at birth. If we agree that healthy gut seeding is important for infants, then this supports the "risk based approach" to GBS (UK standard of care) as opposed to universal use of antibiotics in labor (US standard of care).
The ongoing Probiotics in Pregnancy (PiP) Study in New Zealand examines whether probiotic supplementation in pregnancy and while breastfeeding can improve long term allergy and eczema in infants.
Not about GBS, but this study from the Pakistan Journal of Medical Sciences found that infant probiotics helped to resolve neonatal jaundice.
Ronnie Falcao has a list of studies relating the microbiome to various aspects of infant health.
